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Lecture # 16 Date _____ Chapter #35~ Plant Structure and Growth Angiosperm structure Three basic organs: Roots (root system) fibrous: mat of thin roots taproot: one large, vertical root Stems (shoot system) nodes: leave attachment internodes: stem segments axillary bud: dormant, vegetative potential terminal bud: apex of young shoot apical dominance: inhibits axillary buds Leaves (shoot system) blade petiole Plant Organ Systems Dermal (epidermis): single layer of cells for protection cuticle Vascular (material transport) xylem: water and dissolved minerals roots to shoots tracheids & vessel elements: xylem elongated cells dead at maturity phloem: food from leaves to roots and fruits sieve-tube members: phloem tubes alive at maturity capped by sieve plates; companion cells (nonconducting) connected by plasmodesmata Ground (photosynthesis, storage, support): pith and cortex Plant Tissue Cell Types Parenchyma primary walls thin and flexible; no secondary walls; large central vacuole; most metabolic functions of plant (chloroplasts) Collenchyma unevenly thick primary walls used for plant support (no secondary walls ; no lignin) Sclerenchyma support element strengthened by secondary cell walls with lignin (may be dead; xylem cells); fibers and sclereids for support Plant Growth Life Cycles annuals: 1 year (wildflowers; food crops) biennials: 2 years (beets; carrots) perennials: many years (trees; shrubs) Meristems apical: tips of roots and buds; primary growth lateral: cylinders of dividing cells along length of roots and stems; secondary growth (wood) Primary growth Roots root cap~ protection of meristem zone of cell division~ primary (apical) meristem zone of elongation~ cells elongate; pushes root tip zone of maturation~ differentiation of cells (formation of 3 tissue systems) Primary Tissues of Roots Stele~ the vascular bundle where both xylem and phloem develop Pith~ central core of stele in monocot; parenchyma cells Cortex~ region of the root between the stele and epidermis (innermost layer: endodermis) Lateral roots~ arise from pericycle (outermost layer of stele); just inside endodermis, cells that may become meristematic Primary Tissues of Stems Vascular bundles (xylem and phloem) Surrounded by ground tissue (xylem faces pith and phloem faces cortex) Mostly parenchyma; some collenchyma and sclerenchyma for support Primary Tissues of Leaves Epidermis/cuticle (protection; desiccation) Stomata (tiny pores for gas exchange and transpiration)/guard cells Mesophyll: ground tissue between upper and lower epidermis (parenchyma with chloroplasts); palisade (most photosynthesis) and spongy (gas circulation) Secondary Growth Two lateral meristems vascular cambium ~ produces secondary xylem (wood) and secondary phloem (diameter increase; annual growth rings) cork cambium ~ produces thick covering that replaces the epidermis; produces cork cells; cork plus cork cambium make up the periderm; lenticels (split regions of periderm) allow for gas exchange; bark~ all tissues external to vascular cambium (phloem plus periderm) Summary of primary & secondary growth in a woody a stem Apical meristem of stem PRIMARY MERISTEMS PRIMARY TISSUES Protoderm Epidermis Primary phloem Procambium LATERAL MERISTEM Secondary phloem Vascular cambium Primary xylem Ground meristem Ground Pith & tissue: Cortex SECONDARY TISSUES Secondary xylem Periderm Cork cambium Cork Lecture #16 Date ______ Chapter 36~ Transport in Plants Transport Overview 1- uptake and loss of water and solutes by individual cells (root cells) 2- short-distance transport from cell to cell (sugar loading from leaves to phloem) 3- long-distance transport of sap within xylem and phloem in whole plant Whole Plant Transport 1- Roots absorb water and dissolved minerals from soil 2- Water and minerals are transported upward from roots to shoots as xylem sap 3- Transpiration, the loss of water from leaves, creates a force that pulls xylem sap upwards 4- Leaves exchange CO2 and O2 through stomata 5- Sugar is produced by photosynthesis in leaves 6- Sugar is transported as phloem sap to roots and other parts of plant 7- Roots exchange gases with air spaces of soil (supports cellular respiration in roots) Cellular Transport Water transport √ Osmosis; hyper-; hypo-; iso Cell wall creates physical pressure: √water potential solutes decrease; pressure increase Water moves from high to low water potential Flaccid (limp, iostonic); Plasmolysis (cell loses water in a hypertonic environment; plasma membrane pulls away); Turgor pressure (influx of water due to osmosis; hypotonic environment) Transport within tissues/organs Tonoplast vacuole membrane Plasmodesmata (components) cytosolic connection Symplast route (lateral) cytoplasmic continuum Apoplast route (lateral) continuum of cell walls Bulk flow (long distance) movement of a fluid by pressure (xylem) Transport of Xylem Sap Transpiration: loss of water vapor from leaves pulls water from roots (transpirational pull); cohesion and adhesion of water Root pressure: at night (low transpiration), roots cells continue to pump minerals into xylem; this generates pressure, pushing sap upwards; guttation Cohesion of Water QuickTime™ and a Cinepak decompressor are needed to see this picture. Transpirational Control Photosynthesis-Transpiration compromise…. Guard cells control the size of the stomata Xerophytes (plants adapted to arid environments)~ thick cuticle; small spines for leaves Translocation of Phloem Sap Translocation: food/phloem transport Sugar source: sugar production organ (mature leaves) Sugar sink: sugar storage organ (growing roots, tips, stems, fruit) 1- loading of sugar into sieve tube at source reduces water potential inside; this causes tube to take up water from surroundings by osmosis 2- this absorption of water generates pressure that forces sap to flow alon tube 3- pressure gradient in tube is reinforced by unloading of sugar and consequent loss of water from tube at the sink 4- xylem then recycles water from sink to source